alsa: fix rate matching in the sequencer

The alsa sequencer rate matching was not actually working correctly.
It would compare the previous queue time with the current time and
compare that to the quantum. This would include uncorrected errors from
jitter and would result in the timeouts being scaled in the wrong
direction forever.

Instead, calculate an ideal queue time and compare our current queue
time against that. We then use the correction to scale the timeout or
the next queue time prediction.

Also use the predicted time as the base time for the event timestamps.
this results in less jitter.

Fixes #3657

spa/plugins/alsa/alsa-seq.c

@@ -703,9 +703,7 @@ static int update_time(struct seq_state *state, uint64_t nsec, bool follower)
 	const snd_seq_real_time_t* queue_time;
 	uint64_t queue_real;
 	double err, corr;
-	uint64_t queue_elapsed;
+	uint64_t q1, q2;
-
-	corr = 1.0 - (state->dll.z2 + state->dll.z3);
 
 	/* take queue time */
 	snd_seq_queue_status_alloca(&status);
@@ -713,33 +711,43 @@ static int update_time(struct seq_state *state, uint64_t nsec, bool follower)
 	queue_time = snd_seq_queue_status_get_real_time(status);
 	queue_real = SPA_TIMESPEC_TO_NSEC(queue_time);
 
-	if (state->queue_time == 0)
-		queue_elapsed = 0;
-	else
-		queue_elapsed = (queue_real - state->queue_time) / corr;
-
-	state->queue_time = queue_real;
-
-	queue_elapsed = NSEC_TO_CLOCK(&state->rate, queue_elapsed);
-
-	err = ((int64_t)state->threshold - (int64_t) queue_elapsed);
-	err = SPA_CLAMP(err, -64, 64);
-
 	if (state->dll.bw == 0.0) {
 		spa_dll_set_bw(&state->dll, SPA_DLL_BW_MAX, state->threshold,
 				state->rate.denom);
 		state->next_time = nsec;
 		state->base_time = nsec;
+		state->queue_next = queue_real;
 	}
+
+	/* track our estimated elapsed time against the real elapsed queue time */
+	q1 = NSEC_TO_CLOCK(&state->rate, state->queue_next);
+	q2 = NSEC_TO_CLOCK(&state->rate, queue_real);
+	err = ((int64_t)q1 - (int64_t) q2);
+
+	if (fabs(err) > state->threshold)
+		spa_dll_init(&state->dll);
+
+	err = SPA_CLAMP(err, -64, 64);
 	corr = spa_dll_update(&state->dll, err);
 
+	/* this is our current estimated queue time and rate */
+	state->queue_time = state->queue_next;
+	state->queue_corr = corr;
+
+	/* make a new estimated queue time with the current quantum, if we are following,
+	 * use the rate correction, else we will use the rate correction only for the new
+	 * timeout. */
+	if (state->following)
+		state->queue_next += state->threshold * corr * 1e9 / state->rate.denom;
+	else
+		state->queue_next += state->threshold * 1e9 / state->rate.denom;
+
 	if ((state->next_time - state->base_time) > BW_PERIOD) {
 		state->base_time = state->next_time;
 		spa_log_debug(state->log, "%p: follower:%d rate:%f bw:%f err:%f (%f %f %f)",
 				state, follower, corr, state->dll.bw, err,
 				state->dll.z1, state->dll.z2, state->dll.z3);
 	}
-
 	state->next_time += state->threshold / corr * 1e9 / state->rate.denom;
 
 	if (!follower && state->clock) {

spa/plugins/alsa/alsa-seq.h

@@ -142,6 +142,8 @@ struct seq_state {
 	uint64_t next_time;
 	uint64_t base_time;
 	uint64_t queue_time;
+	uint64_t queue_next;
+	double queue_corr;
 
 	unsigned int opened:1;
 	unsigned int started:1;